Upset when not opening joint (5)
I believe the answer is:
ankle
'joint' is the definition.
(joint of the body)
'upset when not opening' is the wordplay.
'upset' becomes 'rankle' (I've seen this before).
'when not opening' means to remove the first letter.
'rankle' with its first letter taken away is 'ANKLE'.
(Other definitions for ankle that I've seen before include "Location of the talus" , "Leg-joint" , "Low joint" , "Joint between foot and leg" , "Lower leg joint" .)